This invention relates to steam generators, particularly small steam generators adapted to produce clean saturated steam to be used in steam cooking, sterilizing apparatus, heat exchangers, and where other needs for clean steam are presented. Typical devices of this type are disclosed in U.S. Pat. Nos. 3,083,288 and 3,114,028, both assigned to the assignee of this application. These generators operate with a supply of fresh potable water, and are used in an open cycle type of operation, where the product steam is piped into a cooker, and after use the steam is exhausted to a drain, usually in condensed form. Thus, there is no condensation of the used steam and recirculation of the condensate to the boiler/generator, as is typically done in larger steam powered generating systems.
Because of this type of duty, the steam generator is supplied with fresh water, and as the demand for steam from the unit continues, an automatic water level control system provides replenishment of water in the boiler tank. It is impractical to treat the water, beyond normal treatment from the potable water supply used to connect to the water inlet of the boiler tank. Thus, in many instances the water supply to such steam generators is relatively hard water, and mineral deposits, especially caked lime, form on the interior of the steam generator tank, particularly during periods of shutdown, and present a continuing maintenance and operation problem.
For example, a typical steam generator of this type is started and used during meal times in a restaurant. It may or may not be shut down between the hours when steam cooking is not required, but in any event the generator will be shut down overnight. In the past, a typical device has been provided with a simple shutoff/blow down valve arrangement, wherein a single control when turned to the off position terminates power to the steam generator and at the same time opens a drain valve. Residual steam pressure was used to "blow down" the generator tank, causing any remaining water to be forced through the drain, with the interior of the tank being surface wet but empty, and open to atmosphere through the drain such that it dried out eventually. Also, many such steam generating units utilize electrical heating elements of the immersion type which project into the tank and are intended to be immersed to the water during operation. Exposure of these elements to air or the steam within the tank, or any other fault which causes the elements to be immersed in caked lime instead of water, results quickly in hot spots being formed in the heating elements with resulting burnout.
Therefore, it has been discovered that blowing down of such steam generator tanks results in the formation of scale on the tank interior, and on the surfaces of the heating elements, at a rather rapid rate, and under conditions such that the scale builds up daily (or nightly) during the shutdown periods. It has been found that in geographic regions where particularly hard water is available from the normal supply, the hardened scale on the tank interiors must be removed regularly, and this entails disassembling the tank, by removing the pressure head, cleaning the scale from the tank interior and the elements, and reassembling. Since these units are subject to the standard boiler codes, the cleaning operations should be performed by qualified personnel, and the task is both difficult and time consuming. When this cleaning operation must be performed once every several months, it becomes a burden to the user.
Furthermore, a typical installation arrangement in the past has been to connect the drain line from the steam generator to an open drain in the kitchen floor. These drains are commonly installed in commercial kitchens, because of the necessary hygenic clean up which is required by health codes. However, in many jurisdictions there is pending or protective legislation which forbids the venting of live steam into an open drain, where the steam might flow back into the kitchen area. Therefore, closed drains will be required by code in many places, even though this is not presently a strict requirement.
It has been known that the build-up of scale within such steam generators can be minimized if the tank is kept full, at least to a level immersing the heating elements, and closed to the atmosphere when the unit is shut down. Steam generators of this type have included provisions to retain water in the tank during overnight periods, or the like, and this in turn minimizes the formation of hardened scale, with the deposits within the tank building up more in the form of a mud or thick but somewhat fluid material. It is believed that provisions have been made to flush the tank upon starting, to assure that these thick but fluid deposits are regularly flushed from the tank. This provides that the deposits do not collect to the point where they interfere with operation of the generator. However, if the generator is started and stopped frequently, automatic flushing at each start is wasteful of water, and also of energy if the water is still hot. Furthermore, if the unit is flushed with cool fresh water while the heater is still hot, it is possible to damage the heater from thermal shock.